Non-circular cross-section coil spring

ABSTRACT

A non-circular cross-section coil spring, wherein a spring wire having a non-circular cross-section shape is coiled into such configuration that a major diameter of the cross-section is directed in the direction intersecting with a center line of the spring. Torsion is preliminarily given to at least a part of the spring wire so that the coil spring inner circumferential surface side of the above-mentioned major diameter may be deviated to a free end side of the spring with reference to the reference plane of extension and contraction of the coil spring in the case where the coil spring is a compression coil spring, but to the side of the aforementioned reference plane of extension and contraction in the case where the coil spring is a tension coil spring. In a coil spring comprising a larger pitch portion and a smaller pitch portion, a large angle of torsion is preliminarily given to the larger pitch portion and a small angle of torsion is preliminarily given to the smaller pitch portion. Upon application of a load, an angle of torsion of the spring wire becomes zero or nearly zero, thereby increase of the maximum shearing stress generated at the inner circumference of the coil spring can be suppressed, and also sharing stress in the spring wire can be made uniform.

BACKGROUND OF THE INVENTION

The present invention relates to a coil spring formed by coiling aspring wire having a non-circular cross-section into a helical shape.

A commonly used coil spring 01 (FIG. 1) is a spring formed by coiling aspring wire 02 having a circular cross-section into a spiral shape. Whenan axial tension load P₁ is exerted upon this coil spring 01, a torsionshearing stress in the direction shown by arrows A₁ is generated in thespring wire 02, and when an axial compression load P₂ is exerted, atorsion shearing stress in the direction shown by arrows A₂ isgenerated. Due to the fact that the spring wire 02 is curved, thistorsion shearing stress becomes maximum at the inner circumference ofthe coil. In this connection, the maximum shearing stress (τ_(max))composed of this maximum shearing stress generated by torsion and thedirect shearing stress generated by the axial load P, is represented bythe following formula:

    τ.sub.max =8DP/nd.sup.3 {(4C-1)/(4C-4)+0.615/C}

where D represents an average diameter of the coil, d represents adiameter of the spring wire, and C (=D/d) represents a spring index.

Due to the fact that this maximum shearing stress (π_(max)) is generatedat the inner circumference of the coil, when an excessive repeated loadis exerted upon the coil spring 01, there is a tendency that cracks maybe generated in the spring wire 02 at the inner circumference of thecoil. In order to obviate this shortcoming, there has been proposed acoil spring 03 (FIG. 2) formed by modifying a cross-section of a springwire 04 into an oval shape and coiling the spring wire 04 with a majordiameter 05 of the oval directed in the direction transverse to a centerline L of the spring (Japanese Utility Model publication No. 27-3261(1952)). In this coil spring 03, shearing stresses at the inner andouter circumferences, respectively, of the coil under action of an axialload are small at the inner circumference and large at the outercircumference as compared to those in a coil spring formed of a springwire having a circular cross-section with a diameter equal to a minordiameter 06 of the spring wire 04, and moreover, owing to reduction ofthe maximum shearing stress (τ_(max)) at the inner circumference of thecoil, a difference between the shearing stresses at the inner and outercircumferences becomes small. Therefore, an energy efficiency of aspring can be improved, and a scope of use is enlarged.

However, in the spring wire 04 of the coil spring 03, due to the factthat a radius of curvature at the inner circumferential portion of thecoil is small, stress distribution on the peripheral surface of thespring wire is uneven, and upon the coiling work it was considereddifficult to coil the spring wire in such manner that the extension lineof the major diameter 05 of the spring wire 04 may intersect with thecenter line L at right angles. In order to obviate this shortcoming,there has been proposed a coil spring 03A, in which the side having asmall radius of curvature of the spring wire 04A of oval shape incross-section is directed towards the outer circumference of the coil(Laid-Open Japanese Patent Specification No. 60-69337 (1985)).

Even in the case of employing such spring wire 04A of oval shape incross-section, if the coiling work is carried out simply in theconventional manner, torsion would be naturally generated in the springwire 04A, and the extension line of the major diameter 05A would notbecome perpendicular to the center line L. However, here it is assumedthat the coil spring 03A shown in FIG. 4 in which the extension line ofthe major diameter 05A intersects with the center line L at rightangles, has been produced. In this case, if an axial load in thedirection of compression is applied to the coil spring 03A, thentorsional forces in the direction shown by arrows B are exerted upon thespring wire 04A in this figure, reference character S designates areference plane of extension and contraction, and the spring wire 04Awould be twisted depending upon the magnitude of the applied forces,resulting in increase of the maximum shearing stress (τ_(max)) under theloaded condition. Whereas, if the axial load is applied in the tensionaldirection, the torsion of the spring wire 04A would be generated in theopposite direction (in the direction shown by arrows C).

In practice, if coiling is carried out in a simple manner as describedabove, the coil springs 03 and 03A both have a tendency that torsion inthe direction shown by arrows B would be naturally generated. FIG. 5 isa diagram showing influences of the naturally generated torsion upon themaximum shearing stress with respect to the coil spring 03, the abscissarepresenting a torsion angle (α) of the spring wire, and the ordinaterepresents a proportion of increase of the maximum shearing stress (%).In the upper portion of FIG. 5 is shown by dash lines the conditionwhere the spring wire 04 of the coil spring 03 has been twisted by anangle (α) from a target coiled attitude (an attitude having theextension of the major diameter 05 intersected at right angles with thecenter line L as shown by solid lines), that is, the condition wheretorsion has been naturally generated as a result of the conventionalcoiling. As will be seen from this figure, if the torsion angle (α)becomes large, the maximum shearing stress (τ_(max)) generated at theinner circumference of the coil spring 03 would be increased.

The direction of this naturally generated torsion is an unfavorabledirection for a compression coil, but is a favorable direction for atension coil. More particularly, in a compression coil, when acompression load that is a load in use is exerted thereupon, a springwire would be twisted up to a far larger angle than the above-mentionednaturally generated torsion, but in a tension coil, when a tension loadthat is a load in use is exerted thereupon, the above-mentionednaturally generated torsion would be restored.

Here, what is to be kept in mind is that in the case where a pitch of acoil spring is not uniform over its entire length but the coil springconsists of a larger pitch portion and a smaller pitch portion, thetorsion generated by application of a load is different between thelarge pitch portion and the small pitch portion, and a torsion angle ofthe large pitch portion is smaller than a torsion angle of the smallpitch portion.

SUMMARY OF THE INVENTION

The present invention has been worked out under the above-mentionedtechnical background, and has it as an object to provide a non-circularcross-section coil spring in which when a load in use has been appliedthereto, increase of the maximum shearing stress generated at the innercircumference of the coil spring is suppressed, and also shearingstresses generated in a spring wire are made uniform.

It is to be noted that the above-mentioned "load in use" implies acompression load if the coil spring is a compression coil spring, but itimplies a tension load if the coil spring is a tension coil spring.

In order to achieve the aforementioned objects, according to the presentinvention, there is provided a non-circular cross-section coil spring inwhich a spring wire having a non-circular cross-section shape is coiledinto such configuration that a major diameter of the cross-section isdirected in the direction intersecting with a center line of the spring,characterized in that torsion is preliminarily given to at least a partof the spring wire so that the coil spring inner circumferential surfaceside of the aforementioned major diameter may be deviated to theopposite side to the direction of application of a load in use withreference to the reference plane of extension and contraction of thecoil spring.

When a compression load has been applied to a coil spring, torsionalforces as shown by arrows B in FIG. 4 are exerted upon the spring wire,but when a tension load has been applied thereto, torsional forces asshown by arrows C in FIG. 4 are exerted upon the spring wire, and thespring wire is twisted in the respective directions. However, accordingto the present invention, since such torsion that the coil spring innercircumference side of the major diameter of the spring wirecross-section may be deviated on the opposite side to the direction ofapplication of the above-mentioned compression load or tension load,that is, the load in use, with reference to the reference plane ofextension and contraction of the coil spring, is preliminarily given tothe spring wire, by appropriately selecting this torsional angle, atorsional angle of the spring wire upon application of a load can bemade zero or nearly zero, and increase of the maximum shearing stress(τ_(max)) generated at the inner circumference of the coil spring can besuppressed.

Also, according to the present invention, there is provided anon-circular cross-section coil spring having a larger pitch portion anda smaller pitch portion in which a spring wire having a non-circularcross-section shape is coiled into such configuration that a majordiameter of the cross-section is directed in the direction intersectingwith a center line of the spring, characterized in that a torsion ispreliminarily given to at least a part of the spring wire so that thecoil spring inner circumferential surface side of the aforementionedmajor diameter may be deviated to the opposite side to the direction ofapplication of a load in use with reference to the reference plane ofextension and contraction of the coil spring, and the angle of theabove-mentioned torsion is larger in the larger pitch portion ascompared to that in the smaller pitch portion.

Since when a load is applied to a coil spring having a larger pitchportion and a smaller pitch portion a larger torsion is generated in thespring wire in the larger pitch portion than in the smaller pitchportion, if the angle of torsion preliminarily given to the larger pitchportion is made larger than the angle of torsion preliminarily given tothe smaller pitch portion as described above, then under a loadedcondition the angle of torsion in the spring wire is made uniform alongthe lengthwise direction of the coil spring, and thereby shearingstresses generated in the coil wire are also made uniform. Furthermore,as described above, by reducing the angle of torsion of the spring wireupon loading to zero or to nearly zero, increase of the maximum shearingstress (τ_(max)) generated at the inner circumference of the coil springcan be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-section view of a known coil spring madeof a spring wire having a circular cross-section;

FIG. 2 and 3, respectively, are partial longitudinal cross-section viewsshowing known coil springs made of spring wires having an ovalcross-section;

FIG. 4 is a partial longitudinal cross-section view showing direction oftorsional forces exerted upon a spring wire when an axial load isapplied to the coil spring in FIG. 3;

FIG. 5 is a diagram showing a relation between an angle of torsion (α)of a spring wire generated upon coiling and a proportion of increase ofthe maximum shearing stress (%);

FIG. 6 is a schematic view showing a manufacturing device of thenon-circular cross-section coil spring according to the presentinvention;

FIG. 7 is a perspective view showing a part of a feed roller pair in themanufacturing device in FIG. 6 as cut along diameters;

FIG. 8 is a perspective view showing wire guide dies in the samemanufacturing device;

FIGS. 9 and 10, respectively, are schematic views showing relationsbetween groove configurations of the feed roller pair and a wirematerial;

FIG. 11 is a diagram showing relations between an inclination angle (θ)of a feed roller groove and an angle of torsion of a coiled spring wire;

FIG. 12 is a partial longitudinal cross-section view showing acompression spring according to one preferred embodiment of the presentinvention;

FIG. 13 is a partial longitudinal cross-section view showing acompression spring according to another preferred embodiment of thepresent invention;

FIG. 14 is a partial longitudinal cross-section view showing a tensionspring according to still another preferred embodiment of the presentinvention; and

FIG. 15 is an enlarged cross-section showing a modification of across-section shape of a spring wire.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

At first, description will be made on a process for making anon-circular cross-section compression coil spring according to thepresent invention (FIG. 6 to 11).

FIG. 6 shows the state where a raw material wire (spring wire) 1 is fedthrough wire guide dies 6 and guide rollers 7 and 8 to coiling rolls 9and 10 by means of a plurality of feed roller pairs 3(3A, 3B), 4(4A, 4B)and 5(5A, 5B) to be given with a predetermined curved configuration, andit is given with a predetermined pitch by a pitch controlling tool 11 tobe formed into a compression coil spring 2. Reference numeral 12designates a cutter for cutting the coil spring into a predeterminedlength. The cross-section shape of the raw material wire 1 is shown inFIG. 9, a portion D of the cross-section is formed into a coil innercircumferential surface, and a portion E thereof is formed into a coilouter circumferential surface. In correspondence to the cross-sectionshape of the raw material wire 1, the grooves of the respective feedroller pairs 3, 4 and 5 and the die hole of the wire guide dies 6 areformed in the same shape (FIGS. 7 and 8). The grooves of the guiderollers 7 and 8 and the grooves of the coiling rolls 9 and 10 are formedin the same shape as the groove of one roller B in FIG. 7.

As shown in FIG. 9, the grooves of the respective feed roller pairs 3, 4and 5 are formed in such manner that the major diameter of the rawmaterial wire 1 may be inclined by an angle (θ) from the plane F whichwould intersect at right angles with the center line of the compressioncoil spring 2 during the coiling process. Assuming that the angle (θ) iszero as shown in FIG. 10, then as described previously an angle oftorsion (α) is generated in the spring wire of the formed coil spring 2.That is, the portion D of the raw material wire 1 is not directedcorrectly toward the center of the spring. FIG. 11 is a diagram showinga relation between the above-mentioned angle of torsion (α) per meter ofthe spring wire 1 and the inclination angle (θ) of the feed rollergrooves with respect to a coil spring having D/d of 5.6, and in thiscoil spring, if the angle (θ) is chosen to be zero as shown in FIG. 10,then the angle of torsion (α) of the spring wire becomes about 21°/m.

As will be seen from FIG. 11, as the inclination angle (θ) of the feedroller grooves becomes large, the angle of torsion of the spring wire ofthe coil spring 2 would become small. And if the inclination angle (θ)ismade sufficiently large, the angle of torsion (α) can be made even minusas shown by dash lines in FIG. 11. In other words, it is possible togive the spring wire a torsion of the opposite direction to the angle oftorsion generated in the case of the conventional coiling. Accordingly,by appropriately selecting the inclination angle (θ) of the feed rollergrooves depending upon a coil pitch and a spring index (C=D/d), adesired angle of torsion can be given to the spring wire of the coilspring 2. A number of examples of the coil spring according to thepresent invention which have been produced in the above-describedmanner, are shown in FIGS. 12 to 14.

A coil spring 2A shown in FIG. 12 is a compression spring having anequal pitch over its entire length, and it is shown under an unloadedcondition. In this compression spring 2A, a coil spring innercircumferential surface side of the major diameter of the cross-sectionof the spring wire is deviated towards the free end side of the springwith reference to the extension/contraction reference plane S (animaginary plane indicating the position where torsion is not generatedin the spring wire upon application of an axial load). Referencecharacter β designates an angle of torsion per one pitch. When an axialcompression load P is applied to the compression coil spring 2A,torsional forces in the directions indicated by arrows B are exertedupon the spring wire, and the angle of torsion β becomes zero or nearlyzero. Therefore, the maximum shearing stress generated at the innercircumferential surface of the coil upon application of a load can besufficiently reduced.

In a compression coil spring 2B shown in FIG. 13 also a coil springinner circumferential surface side of the major diameter of thecross-section of the spring wire is deviated towards the free end sideof the spring with reference to the extension/contraction referenceplane S. However, this compression coil spring 2B is provided with alarger pitch portion where an angle of torsion per one pitch isrepresented by γ and a smaller pitch portion where an angle of torsionper one pitch is represented by δ, and upon unloading the angle oftorsion γ in the larger pitch portion is set larger than the angle oftortion δ in the smaller pitch portion. When a compression load P isapplied to the compression coil spring 2B, the spring wire would betwisted in the direction shown by arrows B, and this angle of torsioncaused by the compression load is large at the larger pitch portion andsmall at the smaller pitch portion. However, since a larger angle oftorsion γ is preliminarily given to the larger pitch portion and a smallangle of torsion δ is given to the smaller pitch portion under a no-loadcondition as described above, under the loaded condition the angles oftorsion γ and δ at the larger pitch portion and the smaller pitchportion would both become zero or nearly zero. Therefore, in thecompression coil spring 2B, upon application of a load the shearingstresses generated in the spring wire would be equalized along thelengthwise direction of the coil, and also the maximum shearing stressgenerated at the inner circumferential surface of the coil can besufficiently reduced.

It is to be noted that in the compression coil spring 2A and 2B, asassisted by the fact that the spring wire has a flat non-circularcross-section shape, a collapsed height of the spring can be reduced.Therefore, these compression coil springs 2A and 2B are especiallysuitable for a coil spring to be used under a compressed conditionrather than under an unloaded free height condition, such as asuspension spring for use in a vehicle.

While description has been made above in connection to compression coilsprings, now description will be made on a tension coil spring. Sincewhen a tension load is applied to a coil spring, the coil wire would betwisted in the opposite direction to the case where a compression loadis applied, the angle of torsion (α) generated in the spring wire bycoiling in the case where the inclination angle (θ) of the grooves ofthe respective feed roller pairs 3, 4 and 5 is made to be zero as shownin FIG. 10, is produced in a favorable direction for a tension coilspring. However, if the above-mentioned angle of torsion (α) is toolarge as compared to the magnitude of the tension load exerted upon thetension coil spring, then even upon application of the tension load,torsion would remain in the spring wire, and hence the angle of torsionwould not become zero or nearly zero. In order to make the angle oftorsion upon application of a tension load to be zero or nearly zero, itis necessary to set the inclination angle (θ) of the grooves of therespective feed roller pairs 3, 4 and 5 at a proper angle, for instance,at about 5°.

On the other hand, in a tension coil spring consisting of a larger pitchportion and a smaller pitch portion, since large torsion is generated ata larger pitch portion as compared to a smaller pitch portion uponapplication of a tension load, when the spring wire is shaped, even ifthe inclination angle (θ) of the respective feed roller pairs 3, 4 and 5is, for instance, 5° upon shaping the larger pitch portion, when thesmaller pitch portion is shaped it is necessary to select theinclination angle (θ) at a larger angle, for instance, at 15°.

A tension coil spring 2C according to the present invention that is oneexample of a tension coil spring formed under the above-mentionedcondition, is shown in FIG. 14. FIG. 14 also shows a state of a coilspring when no load is applied thereto. In this tension coil spring 2C,the coil inner circumferential surface side of a major diameter of across-section of a spring wire is deviated by a proper torsion angle onthe opposite side to the direction of application of a tension loadindicated by arrows P with reference to the extension/contractionreference plane S, that is, on the side of the extension/contractionreference plane S. Reference character γ represents a torsion angle perone pitch at the larger pitch portion, and reference character δrepresents a torsion angle per one pitch at the smaller pitch portion(γ>δ). If a tension load is applied to the tension coil spring 2C,torsional forces in the directions indicated by arrows C are exertedupon the spring wire, and the variation of the torsion angle γ or δcaused by the above-mentioned torsional forces is large at the largerpitch portion, and is small at the smaller pitch portion. Accordingly,the torsion angles γ and δ of the spring wire both become zero or nearlyzero, as a result, the shearing stresses generated in the spring wireare equalized along the lengthwise direction of the coil spring, andalso the maximum shearing stress generated at the coil innercircumferential surface can be sufficiently reduced.

In a spring wire 12 shown in FIG. 15, similarly to the spring wires inthe above-described respective embodiments, the basic cross-sectionshape is an oval shape consisting of a semi-circle and a semi-ellipse,but the semi-circle portion has recesses 13, 13. Advantages in the caseof forming a coil spring according to the present to the presentinvention by making use of the spring wire 12, are that during thecoiling work, the raw material wire can be surely gripped between feedrollers formed with conformed grooves (grooves having projectionsconformed with the recesses 13) so as not to rotate, and that it ispossible to easily give a desired angle of torsion to the spring wire12. Such an effect and advantage can be obtained, even if the recess 13is provided only one or even if a flat cut plane is provided in place ofthe recess 13.

What is claimed is:
 1. A coil spring having a center line, wherein aspring wire having a non-circular cross-section with a major diameterand a minor diameter is coiled into a spring configuration with saidmajor diameter of said cross-section in a direction intersecting saidcenter line of said spring; characterized in that said coil springincludes a preliminary torsion, applied to said spring wire when saidcoil spring is formed, such that a line parallel to said major diameterand a plane perpendicular to said center line form an angle facing saidcenter line, wherein said parallel line forming a side of said angle isinclined toward a direction opposite to the direction of application ofa load to be applied in use of said spring and the degree of saidapplied preliminary torsion is varied in said coil spring relative to areference plane of extension and contraction of said coil spring.
 2. Acoil spring as claimed in claim 1, wherein said coil spring comprises alarger pitch portion and a smaller pitch portion, and said preliminarytorsion applied to said larger pitch portion is larger than saidpreliminary torsion applied to said smaller pitch portion.
 3. A coilspring as claimed in claim 1 or 2, wherein said load to be applied inuse of said spring is a compression load.
 4. A coil spring as claimed inclaim 1 or 2, wherein said load to be applied in use of said spring is atension load.